What is this test?
Protein S works with protein C to limit clotting. The free fraction is the part available for this role, but laboratories may use an antigen assay or a functional activity assay, which answer related but different questions.
Why might it be ordered?
- For specialist assessment of possible protein S deficiency after a clot.
- For selected family investigations when a thrombosis specialist recommends testing.
What sample is needed?
A blood sample is required. The laboratory chooses the correct tube and handles the sample for this test.
How should you prepare?
The published schedule states receipt by Thursday 18:00 and a result on Monday after 18:00; confirm current timing. Pregnancy, oestrogen, acute thrombosis, liver disease and anticoagulants can alter protein S. Never stop treatment on your own.
How should you understand the result?
Confirm whether the report is free protein S antigen or functional activity. A low result may be acquired or temporary and often needs repeat specialist testing before inherited deficiency is diagnosed.
Sources and editorial status
This patient-facing text is an editorial draft. It should be approved by a laboratory physician before official publication.